[Instant] Add link to Duration page

Now Instant contains a link to Duration as well as vice-versa.

lib/Instant.pod

@@ -11,7 +11,7 @@ An C<Instant> can be used to create a C<DateTime> object set to that
 C<Instant>. The pseudo-constant C<now> returns the current time as an
 C<Instant>.
 
-Basic math is defined for C<Instant>s (as well as C<Duration>s). Adding an
+Basic math is defined for C<Instant>s (as well as L<Duration>s). Adding an
 C<Instant> to a C<Duration> returns another Instant. Subtracting two C<Instant>s
 will yield a C<Duration>. Adding two C<Instant>s is explicitly disallowed. All
 other operations with Instants are undefined.